High-voltage direct current (DC) circuit breaker is one of crucial devices in multi-end. DC power transmission systems. A multi-end high-voltage DC power transmission system has a high voltage level and small line impedance, and once a line short-circuit fault occurs, a DC power transmission network and an alternating current (AC) network will be momentarily affected, therefore, the fault must be cleared quickly. Therefore, a high-voltage DC circuit breaker needs to act as quickly as possible to shorten the duration of the fault or suppress the fault current to the greatest extent, so as to reduce the impacts of the fault on the AC/DC power transmission network. Because the high-voltage DC circuit breaker is connected in series in the power transmission line, the power flow direction in the power transmission line is uncertain, and the current may flow in one of two directions. Therefore, the circuit breaker is required to have the capability of breaking DC currents in two directions.
Chinese Patent Application No. CN102780200A uses a conventional high-voltage DC circuit breaker to break a DC current, where the structure of the conventional high-voltage DC circuit breaker is formed by three parts: an AC circuit breaker, an LC oscillation loop, and a power-consuming element. After the AC circuit breaker is open, an arc is generated. The voltage of the arc oscillates with the LC oscillation loop. When the peak value of the oscillation current reaches the amplitude of the DC current, the DC current can be cancelled completely, so that a zero-crossing point appears at the port of the circuit breaker, which causes the arc to extinguish, thereby achieving an objective of turning off the DC current. Such a breaking method does not include a power semiconductor device, is not directional, and therefore can break currents in two directions, and has a small loss during, normal operation. However, the conventional high-voltage DC circuit breaker requires a long arc-extinguishing time of about a few tens of milliseconds, and cannot meet the requirement for quick fault isolation in the multi-end DC power transmission system.
To quickly isolate of the DC fault current and maintain high power transmission efficiency, Chinese Patent Application No. CN1026877221A discloses an apparatus and method for short-circuiting a current of a power transmission line power distribution line, and a current limiting arrangement, where the apparatus includes a primary circuit breaker, a high-speed switch, an auxiliary circuit breaker, and a non-linear resistance power-consuming element. In a normal operation mode, the current of the line flows through an auxiliary loop, with a small conduction loss; in a fault mode, the current is switched to the primary circuit breaker, and is finally absorbed by the power-consuming element, thereby breaking the current.
After the high-voltage DC breaker apparatus turns off the fault current, the primary circuit breaker bears a voltage of a few hundreds of kilovolts. The number of power semiconductor devices connected in series in one current direction is up to a few hundreds. The power semiconductor device can be conducted in only one direction; therefore, in order to turn off the fault current in both the two current directions, the basic series unit of the primary circuit breaker in the high-voltage DC breaker apparatus adopts two anti-series or anti-parallel structures of power semiconductor devices, and the number of power semiconductor devices in the primary circuit breaker is doubled. When the current is broken in a first current direction, the power semiconductor devices in a second current direction do not have any beneficial effect to break the current or bear the voltage, that is, the utilization rate of the power semiconductor devices of the primary circuit breaker is only 50%. As the costs of the power semiconductor devices account for a large proportion of the total costs of the apparatus, providing the function of breaking currents in two directions will greatly increase the costs of the apparatus. The arrangement of the power semiconductor devices in the second current direction in the primary circuit breaker does not have any beneficial effect, and over-voltage and over-current generated When the current is turned off in the first current direction have adverse effects on the power semiconductor devices in the second current direction. If the power semiconductor devices in the second current direction and the power semiconductor devices in the first current direction are connected in an anti-parallel manner, an over-voltage generated when the current is turned off in the first current direction will be applied to the power semiconductor devices in the second current direction, and the voltage is a reverse voltage for the power semiconductor devices in the second current direction and will cause damage to the devices; if the power semiconductor devices with anti-parallel diodes in the second current direction and the power semiconductor devices with anti-parallel diodes in the first current direction are connected in an anti-series manner, an abruptly increased current generated when the current is turned off in the first current direction will flow through free-wheeling diodes in the power semiconductor devices in the second current direction, reducing the service life of the devices.
The arrangement of the power semiconductor devices in the second current direction also has an adverse effect on the structural design and electrical design of the primary circuit breaker. The arrangement directions of the power semiconductor devices in the first current direction are consistent, making the electrical design and structural design have consistency. The arrangement of the power semiconductor devices in the second current direction destroys the original consistency in the arrangement direction, increasing the difficulty in device layout, installation and wiring.
In the auxiliary circuit breaker branch of Chinese Patent Application No. CN102687221A, the ultra-high-speed mechanical switch is connected in parallel to the primary circuit breaker, the ultra-high-speed mechanical switch cannot completely isolate the apparatus to protect the primary circuit breaker, and the primary circuit breaker does not have obvious breakpoints and is difficult to repair and maintain.